Polymer electrolytes crosslinked by e-beam

ABSTRACT

A method of making a crosslinked polymer is provided as well as the polymer so made, the method comprising the steps of: providing a highly fluorinated fluoropolymer, typically a perfluorinated fluoropolymer, comprising pendent groups which include a group according to the formula —SO 2 X, where X is F, Cl, Br, OH, or —O − M + , where M +  is a monovalent cation, and exposing said fluoropolymer to electron beam radiation so as to result in the formation of crosslinks. Typically, the method according to the present invention additionally comprises the step of: forming said fluoropolymer into a membrane, typically having a thickness of 90 microns or less, more typically 60 microns or less, and most typically 30 microns or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 10/712,361, filed Nov.13, 2003, now allowed, the disclosure of which is incorporated byreference in its entirety herein.

FIELD OF THE INVENTION

This invention relates to a method of making a crosslinked polymer andthe polymer so made, the method comprising the steps of: providing ahighly fluorinated fluoropolymer, typically a perfluorinatedfluoropolymer, comprising pendent groups which include a group accordingto the formula —SO₂X, where X is F, Cl, Br, OH, or —O⁻ M⁺, where M⁺ is amonovalent cation, and exposing said fluoropolymer to electron beamradiation so as to result in the formation of crosslinks.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,230,549 purportedly discloses polymer membranes to beused in electrochemical cells produced by radiation grafting techniques.

U.S. Pat. Nos. 6,225,368 and 6,387,964 purportedly disclosemonomer-grafted cross-linked polymers made by radiation cross-linkingand grafting. In some embodiments, the monomer-grafted cross-linkedpolymer may be a fluoropolymer. In some embodiments, the monomer-graftedcross-linked polymer may then be sulfonated and used as an ion-exchangemembrane in an electrochemical cell.

U.S. Pat. No. 6,255,370 purportedly discloses a solid polyelectrolytefuel cell comprising a solid polyelectrolyte membrane, where the watercontent of the solid polyelectrolyte membrane is greater adjacent to thenegative electrode. In one aspect, water content is purportedlycontrolled by controlling the degree of crosslinking in the membrane.The reference states, “when side chains are introduced into the film ofa main chain copolymer, the material for the side chains or thecrosslinking material is contacted with only one surface of the film,whereby the concentration of the side chains thus formed in the film orthe degree of crosslinking in the film may be controlled in the intendedmanner.” ('370, col. 5, lns. 57-61). Such treatment is followed bysulfonation. ('370, col. 6, lns. 31-48).

U.S. Pat. No. 5,260,351 purportedly discloses perfluoroelastomers curedby radiation in the absence of curing agents. The reference purportedlyrelates to curing of fully fluorinated polymers, such as those preparedfrom tetrafluoroethylene, a perfluoralkyl perfluorovinyl ether, and curesite or crosslinking units providing at least one of nitrile,perfluorophenyl, bromine or iodine in the resulting terpolymer.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a method of making a crosslinkedpolymer comprising the steps of: providing a highly fluorinatedfluoropolymer, typically a perfluorinated fluoropolymer, comprisingpendent groups which include a group according to the formula —SO₂X,where X is F, Cl, Br, OH, or —O⁻M⁺, where M⁺ is a monovalent cation, andexposing said fluoropolymer to electron beam radiation so as to resultin the formation of crosslinks. The pendant groups are typicallyaccording to the formula —R¹—SO₂X, where R¹ is a branched or unbranchedperfluoroalkyl or perfluoroether group comprising 1-15 carbon atoms and0-4 oxygen atoms, and most typically —O—(CF₂)₄—SO₂X. Typically, themethod according to the present invention additionally comprises thestep of: forming said fluoropolymer into a membrane, typically having athickness of 90 microns or less, more typically 60 microns or less, andmost typically 30 microns or less. Typically, the electron beamradiation is in a dose of 4 Mrad or more, more typically 5 Mrad or more,and most typically 6 Mrad or more. Typically, the electron beamradiation is in a dose of less than 14 Mrad, and more typically lessthan 10 Mrad.

In another aspect, the present invention provides crosslinked polymersmade according to any of the methods of the present invention.

What has not been described in the art, and is provided by the presentinvention, is a method of crosslinking a polymer comprising pendentgroups which include a group according to the formula —SO₂X, where X isF, Cl, Br, OH, or —O⁻M⁺, typically a membrane for use as a polymerelectrolyte membrane, using electron beam radiation.

In this application:

“equivalent weight” (EW) of a polymer means the weight of polymer whichwill neutralize one equivalent of base;

“hydration product” (HP) of a polymer means the number of equivalents(moles) of water absorbed by a membrane per equivalent of sulfonic acidgroups present in the membrane multiplied by the equivalent weight ofthe polymer; and

“highly fluorinated” means containing fluorine in an amount of 40 wt %or more, typically 50 wt % or more and more typically 60 wt % or more.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing dynamic mechanical analysis (DMA) results fortwo comparative polymers (A and B) and one polymer according to thepresent invention (C).

FIG. 2 is a graph showing Tg for two comparative polymers (0 Mrad and 2Mrad) and one polymer according to the present invention (6 Mrad).

DETAILED DESCRIPTION

The present invention provides a method of making a crosslinked polymer.The polymer to be crosslinked comprises pendent groups which include agroup according to the formula —SO₂X, where X is F, Cl, Br, OH, or—O⁻M⁺, where M⁺ is a monovalent cation, typically an alkali metal cationsuch as Na⁺, but most typically OH. Such polymers may be useful in themanufacture of polymer electrolyte membranes (PEM's), such as are usedin electrolytic cells such as fuel cells.

PEM's manufactured from the crosslinked polymer according to the presentinvention may be used in the fabrication of membrane electrodeassemblies (MEA's) for use in fuel cells. An MEA is the central elementof a proton exchange membrane fuel cell, such as a hydrogen fuel cell.Fuel cells are electrochemical cells which produce usable electricity bythe catalyzed combination of a fuel such as hydrogen and an oxidant suchas oxygen. Typical MEA's comprise a polymer electrolyte membrane (PEM)(also known as an ion conductive membrane (ICM)), which functions as asolid electrolyte. One face of the PEM is in contact with an anodeelectrode layer and the opposite face is in contact with a cathodeelectrode layer. Each electrode layer includes electrochemicalcatalysts, typically including platinum metal. Gas diffusion layers(GDL's) facilitate gas transport to and from the anode and cathodeelectrode materials and conduct electrical current. The GDL may also becalled a fluid transport layer (FTL) or a diffuser/current collector(DCC). The anode and cathode electrode layers may be applied to GDL's inthe form of a catalyst ink, and the resulting coated GDL's sandwichedwith a PEM to form a five-layer MEA. Alternately, the anode and cathodeelectrode layers may be applied to opposite sides of the PEM in the formof a catalyst ink, and the resulting catalyst-coated membrane (CCM)sandwiched with two GDL's to form a five-layer MEA. The five layers of afive-layer MEA are, in order: anode GDL, anode electrode layer, PEM,cathode electrode layer, and cathode GDL. In a typical PEM fuel cell,protons are formed at the anode via hydrogen oxidation and transportedacross the PEM to the cathode to react with oxygen, causing electricalcurrent to flow in an external circuit connecting the electrodes. ThePEM forms a durable, non-porous, electrically non-conductive mechanicalbarrier between the reactant gases, yet it also passes H⁺ions readily.

The polymer to be crosslinked comprises a backbone, which may bebranched or unbranched but is typically unbranched. The backbone ishighly fluorinated and more typically perfluorinated. The backbone maycomprise units derived from tetrafluoroethylene (TFE) and units derivedfrom co-monomers, typically including at least one according to theformula CF₂═CY—R where Y is typically F but may also be CF₃, and where Ris a pendent group which includes a group according to the formula—SO₂X, where X is F, Cl, Br, OH, or —O⁻M⁺, where M⁺ is a monovalentcation, typically an alkali metal cation such as Na⁺. X is mosttypically OH. In an alternative embodiment, side groups R may be addedto the backbone by grafting. Typically, side groups R are highlyfluorinated and more typically perfluorinated. R may be aromatic ornon-aromatic. Typically, R is —R¹—SO₂X, where R¹ is a branched orunbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbonatoms and 0-4 oxygen atoms. R¹ is typically —O—R²— wherein R² is abranched or unbranched perfluoroalkyl or perfluoroether group comprising1-15 carbon atoms and 0-4 oxygen atoms. R¹ is more typically —O—R³—wherein R³ is a perfluoroalkyl group comprising 1-15 carbon atoms.

Examples of R¹ include:

—(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or15

(—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5

(—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4, or 5(—CF₂CF(CF₃)—)_(n)—CF₂—where n is 1, 2, 3 or 4

(—O—CF₂CF₂—)_(n) where n is 1, 2, 3, 4, 5, 6 or 7

(—O—CF₂CF₂CF₂—)_(n) where n is 1, 2, 3, 4, or 5

(—O—CF₂CF₂CF₂CF₂—)_(n) where n is 1, 2 or 3

(—O—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5

(—O—CF₂CF(CF₂CF₃)—)_(n) where n is 1, 2 or 3

(—O—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4 or 5

(—O—CF(CF₂CF₃)CF₂—)_(n) where n is 1, 2 or 3

(—O—CF₂CF(CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4

(—O—CF₂CF(CF₂CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3

(—O—CF(CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4

(—O—CF(CF₂CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3

—O—(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 or 14

R is typically —O—CF₂CF₂CF₂CF₂—SO₂X or —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X andmost typically —O—CF₂CF₂CF₂CF₂—SO₂X, where X is F, Cl, Br, OH, or —O⁻M⁺,but most typically OH.

The fluoromonomer according to formula I may be synthesized by anysuitable means, including methods disclosed in U.S. Pat. No. 6,624,328.

The polymer to be crosslinked may be made by any suitable method,including emulsion polymerization, extrusion polymerization,polymerization in supercritical carbon dioxide, solution or suspensionpolymerization, and the like, which may be batchwise or continuous.

The acid-functional pendent groups typically are present in an amountsufficient to result in an hydration product (HP) of greater than15,000, more typically greater than 18,000, more typically greater than22,000, and most typically greater than 25,000. In general, higher HPcorrelates with higher ionic conductance.

The acid-functional pendent groups typically are present in an amountsufficient to result in an equivalent weight (EW) of less than 1200,more typically less than 1100, and more typically less than 1000, andmore typically less than 900.

Typically, the polymer is formed into a membrane prior to crosslinking.Any suitable method of forming the membrane may be used. The polymer istypically cast from a suspension. Any suitable casting method may beused, including bar coating, spray coating, slit coating, brush coating,and the like. Alternately, the membrane may be formed from neat polymerin a melt process such as extrusion. After forming, the membrane may beannealed, typically at a temperature of 120° C. or higher, moretypically 130° C. or higher, most typically 150° C. or higher. Typicallythe membrane has a thickness of 90 microns or less, more typically 60microns or less, and most typically 30 microns or less. A thinnermembrane may provide less resistance to the passage of ions. In fuelcell use, this may result in cooler operation and greater output ofusable energy. Thinner membranes must be made of materials that maintaintheir structural integrity in use.

The step of crosslinking comprises the step of exposing thefluoropolymer to electron beam radiation so as to result in theformation of crosslinks. Typically, the electron beam radiation is in adose of 4 Mrad or more, more typically 5 Mrad or more, and mosttypically 6 Mrad or more. Typically, the electron beam radiation is in adose of less than 14 Mrad and more typically less than 10 Mrad. Anysuitable apparatus may be used. A continuous process of exposure may beused to treat roll good membranes.

Optionally a crosslinking agent may be added. The crosslinking agent maybe added by any suitable method, including blending with the polymerbefore forming into a membrane and application of the crosslinking agentto the membrane, e.g. by immersion in a solution of the crosslinkingagent. Typical agents may include multifunctional compounds such asmultifunctional alkenes, multifunctional acrylates, multifunctionalvinyl ethers, and the like, which may be non-fluorinated or fluorinatedto a low level but which are more typically highly fluorinated and moretypically perfluorinated.

In a further embodiment, the polymer may be imbibed into a poroussupporting matrix prior to crosslinking, typically in the form of a thinmembrane having a thickness of 90 microns or less, more typically 60microns or less, and most typically 30 microns or less. Any suitablemethod of imbibing the polymer into the pores of the supporting matrixmay be used, including overpressure, vacuum, wicking, immersion, and thelike. The blend becomes embedded in the matrix upon crosslinking. Anysuitable supporting matrix may be used. Typically the supporting matrixis electrically non-conductive. Typically, the supporting matrix iscomposed of a fluoropolymer, which is more typically perfluorinated.Typical matrices include porous polytetrafluoroethylene (PTFE), such asbiaxially stretched PTFE webs.

It will be understood that polymers and membranes made according to themethod of the present invention may differ in chemical structure fromthose made by other methods, in the placement of crosslinks, theplacement of acid-functional groups, the presence or absence ofcrosslinks on pendent groups or of acid-functional groups on crosslinks,and the like.

This invention is useful in the manufacture of strengthened polymerelectrolyte membranes for use in electrolytic cells such as fuel cells.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLES

Unless otherwise noted, all reagents were obtained or are available fromAldrich Chemical Co., Milwaukee, Wis., or may be synthesized by knownmethods.

Polymer

The polymer electrolyte used in the present examples was made byemulsion co-polymerization of tetrafluoroethylene (TFE) withCF₂═CF—O—(CF₂)₄—SO₂F (MV4S), which was synthesized by the methoddisclosed in U.S. Pat. No. 6,624,328, the disclosure of which isincorporated herein by reference.

MV4S was preemulsified in water with APFO emulsifier (ammoniumperfluorooctanoate, C₇F₁₅COONH₄) under high shear (24,000 rpm), using anULTRA-TURRAX® Model T 25 disperser S25KV-25F (IKA-Werke GmbH & Co. KG,Staufen, Germany) for 2 min. A polymerization kettle equipped with animpeller agitator system was charged with deionized water. The kettlewas heated up to 50° C. and then the preemulsion was charged into theoxygen-free polymerization kettle. At 50° C. the kettle was furthercharged with gaseous tetrafluoroethylene (TFE) to 6 bar absolutereaction pressure. At 50° C. and 240 rpm agitator speed thepolymerization was initiated by addition of sodium disulfite andammonium peroxodisulfate. During the course of the reaction, thereaction temperature was maintained at 50° C. Reaction pressure wasmaintained at 6 bar absolute by feeding additional TFE into the gasphase. A second portion of MV4S-preemulsion was prepared, as describedabove. The second preemulsion portion was fed continuously into theliquid phase during the course of the reaction.

After feeding additional TFE, the monomer valve was closed and themonomer feed interrupted. The continuing polymerization reduced thepressure of the monomer gas phase to about 2 bar. At that time, thereactor was vented and flushed with nitrogen gas.

The polymer dispersion thus obtained was mixed with 2-3 equivalents ofLiOH and 2 equivalents of Li₂CO₃ (equivalents based on sulfonyl fluorideconcentration) and enough water to make a 20% polymer solids mixture.This mixture was heated to 250° C. for four hours. Most (>95%) of thepolymer became dispersed under these conditions. The dispersions werefiltered to remove LiF and undispersed polymer, and then ion exchangedon Mitsubishi Diaion SKT10L ion exchange resin to give the acid form ofthe ionomer. The resulting polymer electrolyte is a perfluorinatedpolymer with acidic side chains according to the formula:—O—(CF₂)₄—SO₃H. The resulting mixture was an acid dispersion at 18 to19% polymer solids. This dispersion was mixed with n-propanol and thenconcentrated in vacu to give the desired 20% solids dispersion in awater/n-propanol solvent mixture of about 30% water/70% n-propanol. Thisbase dispersion was used to cast membranes.

Membranes

Polymer membrane samples for testing were cast by knife coating out of awater/propanol suspension (40% water/60% n-propanol) containing 20%solids onto a glass plate, dried at 80° C. for 10 minutes, and annealedat 200° C. for 10 minutes. The thickness of the resulting films wasapproximately 30 microns. The films were then removed from the glassplate, cut into strips, placed in polyethylene bags and purged withnitrogen.

E-Beam

The membrane samples were exposed to an e-beam source. (Energy SciencesCB300, Energy Sciences, Inc., Wilmington, Mass.). The dose wascontrolled to 2 Mrad per pass. Samples were subjected to 0, 1 or 3passes for a total e-beam dose of 0, 2 or 6 Mrad.

Analysis

Tg was measured by dynamic mechanical analysis (DMA) for the samplesexposed to e-beam doses of 0, 2 or 6 Mrad. In DMA, a sample of a polymerto be tested is clamped in a test apparatus that applies an oscillatingforce and measures the resulting displacement of the sample. The processis carried out in a temperature controlled environment. Temperature isramped upward as measurements are taken. From this data, the apparatustypically calculates, records and displays the elastic modulus (E′),loss modulus (E″), and damping factor (tan delta) of the sample as afunction of temperature. Tg is taken to be the maximum in tan delta.

In the present examples, a Rheometrics Solid Analyzer RSA II (TAInstruments, New Castle, Del., USA) was used at a frequency of 1 Hertz(6.28 rad/sec). A thin strip of sample was tested, measuring about 6.5mm wide by about 25 mm long. Measurements were taken under tension overthe temperature range of 25° C. to 200° C.

FIG. 1 is a graph showing DMA results at each dose. Trace A represents 0Mrad (Comparative), trace B represents 2 Mrad (Comparative), and trace Crepresents 6 Mrad (Invention). FIG. 2 is a graph showing Tg at eachdose, where Tg is taken as a maximum in the tan delta data representedin FIG. 1. Tg is elevated for the sample exposed to 6 Mrad of e-beamradiation, indicating that crosslinking has occurred.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

1. A method of making a crosslinked polymer comprising the steps of: a)providing a highly fluorinated fluoropolymer comprising pendent groupswhich include a group according to the formula —SO₂X, where X is F, Cl,Br, OH or —O⁻M⁺, where M⁺ is a monovalent cation; and b) exposing saidfluoropolymer to electron beam radiation so as to result in theformation of crosslinks.
 2. The method according to claim 1 wherein saidmethod additionally comprises, prior to said step b), the step of: c)forming said fluoropolymer into a membrane.
 3. The method according toclaim 2 wherein said membrane has a thickness of 90 microns or less. 4.The method according to claim 1 wherein said step of exposing saidfluoropolymer to electron beam radiation comprises exposing saidfluoropolymer to greater than 4 Mrad of electron beam radiation.
 5. Themethod according to claim 1 wherein said highly fluorinatedfluoropolymer is perfluorinated.
 6. The method according to claim 1wherein said pendent groups are according to the formula —R¹—SO₂X, whereR¹ is a branched or unbranched perfluoroalkyl or perfluoroether groupcomprising 1-15 carbon atoms and 0-4 oxygen atoms, and where X is F, Cl,Br, OH or —O⁻M⁺, where M⁺ is a monovalent cation.
 7. The methodaccording to claim 1 wherein said pendent groups are groups according tothe formula —O—(CF₂)₄—SO₂X, where X is F, Cl, Br, OH or —O⁻M⁺, where M⁺is a monovalent cation.
 8. The method according to claim 7 wherein X isOH.
 9. The method according to claim 2 wherein said pendent groups areaccording to the formula —R¹—SO₂X, where R¹ is a branched or unbranchedperfluoroalkyl or perfluoroether group comprising 1-15 carbon atoms and0-4 oxygen atoms, and where X is F, Cl, Br, OH or —O⁻M⁺, where M⁺ is amonovalent cation.
 10. The method according to claim 2 wherein saidpendent groups are groups according to the formula —O—(CF₂)₄—SO₂X, whereX is F, Cl, Br, OH or —O⁻M⁺, where M⁺ is a monovalent cation.
 11. Themethod according to claim 10 wherein X is OH.
 12. The method accordingto claim 3 wherein said pendent groups are according to the formula—R¹—SO₂X, where R¹ is a branched or unbranched perfluoroalkyl orperfluoroether group comprising 1-15 carbon atoms and 0-4 oxygen atoms,and where X is F, Cl, Br, OH or —O⁻M⁺, where M⁺ is a monovalent cation.13. The method according to claim 3 wherein said pendent groups aregroups according to the formula —O—(CF₂)₄—SO₂X, where X is F, Cl, Br, OHor —O⁻M⁺, where M⁺ is a monovalent cation.
 14. The method according toclaim 13 wherein X is OH.
 15. The method according to claim 4 whereinsaid pendent groups are according to the formula —R¹—SO₂X, where R¹ is abranched or unbranched perfluoroalkyl or perfluoroether group comprising1-15 carbon atoms and 0-4 oxygen atoms, and where X is F, Cl, Br, OH or—O⁻M⁺, where M⁺ is a monovalent cation.
 16. The method according toclaim 4 wherein said pendent groups are groups according to the formula—O—(CF₂)₄—SO₂X, where X is F, Cl, Br, OH or —O⁻M⁺, where M⁺ is amonovalent cation.
 17. The method according to claim 16 wherein X is OH.18. The method according to claim 1 wherein step c) comprises imbibingsaid fluoropolymer into a porous supporting matrix.
 19. The methodaccording to claim 18 wherein said porous supporting matrix is a porouspolytetrafluoroethylene web.
 20. A polymer electrolyte membranecomprising the crosslinked polymer made according to the method ofclaim
 1. 21. A polymer electrolyte membrane comprising the crosslinkedpolymer made according to the method of claim
 2. 22. A polymerelectrolyte membrane comprising the crosslinked polymer made accordingto the method of claim
 3. 23. A polymer electrolyte membrane comprisingthe crosslinked polymer made according to the method of claim
 4. 24. Apolymer electrolyte membrane comprising the crosslinked polymer madeaccording to the method of claim
 5. 25. A polymer electrolyte membranecomprising the crosslinked polymer made according to the method of claim6.
 26. A polymer electrolyte membrane comprising the crosslinked polymermade according to the method of claim
 7. 27. A polymer electrolytemembrane comprising the crosslinked polymer made according to the methodof claim
 8. 28. A polymer electrolyte membrane comprising thecrosslinked polymer made according to the method of claim
 9. 29. Apolymer electrolyte membrane comprising the crosslinked polymer madeaccording to the method of claim
 10. 30. A polymer electrolyte membranecomprising the crosslinked polymer made according to the method of claim11.
 31. A polymer electrolyte membrane comprising the crosslinkedpolymer made according to the method of claim
 12. 32. A polymerelectrolyte membrane comprising the crosslinked polymer made accordingto the method of claim
 13. 33. A polymer electrolyte membrane comprisingthe crosslinked polymer made according to the method of claim
 14. 34. Apolymer electrolyte membrane comprising the crosslinked polymer madeaccording to the method of claim
 15. 35. A polymer electrolyte membranecomprising the crosslinked polymer made according to the method of claim16.
 36. A polymer electrolyte membrane comprising the crosslinkedpolymer made according to the method of claim
 17. 37. A polymerelectrolyte membrane comprising the crosslinked polymer made accordingto the method of claim
 18. 38. A polymer electrolyte membrane comprisingthe crosslinked polymer made according to the method of claim 19.